The invention relates to a steam generator, in particular a waste-heat steam generator, including an elongate gas flue having an inflow end for hot gas and an outflow end for cooled hot gas, a first evaporator heating surface, a first feed water supply line throughflow-connected to the first evaporator heating surface on an inlet side, a second evaporator heating surface disposed in the gas flue downstream of the first evaporator heating surface, as seen in a throughflow direction of the hot gas, the second evaporator heating surface throughflow-connected both on an inlet and on an outlet side to a low-pressure drum, and a second feed water supply line connected to the low-pressure drum. The first supply line has a first treatment point for adding and/or extracting chemicals for water treatment and the second supply line has a second treatment point for adding and/or extracting chemicals for water treatment. The invention also relates to a method for operating the steam generator.
Such a steam generator is known from ASME Paper 97-GT337 entitled xe2x80x9cOnce-through Heat Recovery Steam Generators working with Sub- and Supercritical Steam Conditions for Combined Cyclesxe2x80x9d, by P. J. Dechamps and J-F. Galopin, International Gas Turbine and Aeroengine Congress and Exhibition, Orlando, Fla. Jun. 2 to Jun. 5, 1997, in particular page 7, left-hand column, second paragraph, and from European Patent Application 0 777 036, corresponding to U.S. Pat. No. 5,776,413.
When the hot gas has high temperatures, the higher pressure stage of that steam generator, that is the pressure stage which is operable in the once-through mode and includes the first evaporator heating surface, also makes it possible to implement high operating pressures up to supercritical states. That allows improved efficiency. Furthermore, the higher pressure stage requires, at most, only a relatively thin-walled water separation bottle at the steam outflow of the first evaporator heating surface, so that faster start-up times of the steam generator and faster reactions of that steam generator to load changes are possible. The second evaporator heating surface, together with the low-pressure drum associated therewith, is operated in the circulating mode and utilizes the low temperature which the hot gas still has after passing through the first evaporator heating surface. That too leads to an improvement in efficiency. Due to the low pressure in the low-pressure drum, the latter can likewise be relatively thin-walled. It is therefore no obstacle either to fast start-up times or to the fast reaction of the steam generator to load changes. In addition, the circulating flow in the second evaporator heating surface has no instabilities and does not lead to any erosions in the second evaporator heating surface.
However, the first evaporator heating surface operated in the once-through mode requires different conditioning of the feed water from that of an evaporator heating surface operated in the circulating mode, that is to say from that of the second evaporator heating surface connected to the low-pressure drum both on the inlet and on the outlet side.
It is accordingly an object of the invention to provide a steam generator, in particular a waste-heat steam generator, and a method for operating the steam generator, which overcome the hereinafore-mentioned disadvantages of the heretofore-known devices and methods of this general type and which ensure that both a first evaporator heating surface operated in a once-through mode and a second evaporator heating surface operated in a circulating mode receive only feed water that is conditioned for them.
With the foregoing and other objects in view there is provided, in accordance with the invention, a steam generator, in particular a waste-heat steam generator, comprising an elongate gas flue having an inflow end for hot gas and an outflow end for cooled hot gas, defining a throughflow direction of the hot gas; a first evaporator heating surface disposed in the gas flue and having an inlet side; a first feed water supply line throughflow-connected to the first evaporator heating surface on the inlet side, the first supply line having a first treatment point for adding and/or extracting chemicals for water treatment, and the first supply line having a preheater heating surface disposed within the gas flue and defining a connection point between the preheater heating surface and the first evaporator heating surface; a second evaporator heating surface disposed in the gas flue downstream of the first evaporator heating surface as seen in the throughflow direction of the hot gas, the second evaporator heating surface having inlet and outlet sides; a low-pressure drum throughflow-connected to the second evaporator heating surface on both the inlet and outlet sides; and a second feed water supply line connected to the low-pressure drum, the second feed water supply line having a second treatment point for adding and/or extracting chemicals for water treatment, and the second supply line throughflow-connected to the first supply line at the connection point.
The feed water for the first evaporator heating surface can be conditioned according to the requirements of this first evaporator heating surface through the use of chemicals which are added to and/or removed from the first treatment point of the first supply line. The feed water for the second evaporator heating surface can be conditioned according to the requirements of the latter by adding and/or removing chemicals at the second treatment point of the second supply line. Both supply lines can therefore be connected to the same common feed-water supply configuration. This feed-water supply configuration can deliver initial feed water of uniform initial quality and may, for example, be a hot well, a feed water treatment plant or a condensate collecting vessel.
In accordance with another feature of the invention, the first treatment point for adding and/or extracting chemicals for water treatment is disposed upstream of the preheater heating surface in terms of throughflow in the first supply line, and the second treatment point for adding and/or extracting chemicals for water treatment is disposed downstream of the connection point to the first supply line in terms of throughflow in the second supply line. Therefore, feed water conditioned in the first supply line may be reconditioned for the second evaporator heating surface.
In accordance with a further feature of the invention, the first supply line has a third treatment point for feed-water treatment, in particular for adding chemicals, downstream of the connection point of the second supply line and upstream of the evaporator heating surface in terms of through low. Therefore, the feed water in the first supply line may be preconditioned for both evaporator heating surfaces through the use of chemicals which are added to the first supply line at the first treatment point. Through the use of chemicals added to the second supply line at the second treatment point, feed water preconditioned for both evaporator heating surfaces from the first supply line may be reconditioned for the second evaporator heating surface. Finally, feed water in the first supply line, which is already preconditioned for both evaporator heating surfaces, may be reconditioned for the first evaporator heating surface at the third treatment point by the addition and/or extraction of further chemicals.
In accordance with an added feature of the invention, the preheater heating surface connected in the first supply line and disposed within the gas flue is a first preheater heating surface; the second supply line has a second preheater heating surface disposed within the gas flue and throughflow-connected to the first supply line at a given connection point upstream of the first preheater heating surface; the first treatment point for at least one of adding and extracting chemicals for water treatment is connected in the first supply line upstream of the first preheater heating surface and downstream of the given connection point in terms of throughflow; and the second treatment point for at least one of adding and extracting chemicals for water treatment is connected in the second supply line upstream of the second preheater heating surface in terms of throughflow. In this way, the feed water for the first evaporator heating surface may be accurately conditioned by the addition and/or extraction of chemicals at the first treatment point and the feed water for the second evaporator heating surface may be accurately conditioned by the addition and/or extraction of chemicals at the second treatment point.
In accordance with an additional feature of the invention, the second treatment point has a deaeration vessel for extracting a gaseous chemical from feed water. The deaeration vessel may advantageously be integrated in the low-pressure drum which is provided with the second supply line for feed water.
With the objects of the invention in view there is also provided a method for operating a steam generator, which comprises adding NH3 and O2 at the first treatment point, and adding NH3 and extracting O2 at the second treatment point.
With the objects of the invention in view, there is additionally provided a method for operating a steam generator, which comprises adding NH3 at the first and second treatment points, and adding O2 at the third treatment point.
With the objects of the invention in view, there is also provided a method for operating a steam generator, which comprises adding NH3 and O2 at the first treatment point, and adding NH3 at the second treatment point.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a steam generator, in particular a waste-heat steam generator, and a method for operating the steam generator, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.